This invention relates to a color transforming method which uses a color gamut compressing technology in converting the color information in an input color space to that in an output color space.
In the conventional closed color processing system using a monolithic input/output device which does not take color management into account, the dynamic range of the signals from the input device have one-to-one correspondence to that of the signals from the output device. This means that the closed color processing system which gives no consideration to color management is capable of spontaneous compression of a color gamut to be reproduced but unable to perform its control.
In a color management system which converts the input signal to a colorimetric signal such as XYZ or L*a*b* before conversion to the output signal, the range of color reproduction has to be taken into account for both input and output sides but, on the other hand, the compression of the color gamut to be reproduced can be controlled. Take, for example, the color processing system shown in FIG. 9; the values of input device dependent data such as RGB that are supplied from the input device are subjected to conversion at the input end so that they are converted to calorimetric data such as L*a*b* which, in turn, is subjected to conversion at the output end so that it is converted to the values of output device dependent data such as CMY before they are delivered to the appropriate output device. The intermediary colorimetric data L*a*b* offers the advantage that the color gamut at the input end can be adjusted to the output color gamut through color gamut compression.
To exploit this advantage of the color gamut compressing technology, various methods have so far been proposed. Por example, Unexamined Published Japanese Patent Application (Kokai) Nos. 253138/1994 and 253139/1994 disclose color gamut compressing techniques that designate different color mapping methods to different subspaces of a color space and which insure continuity between the respective subspaces. In order to insure the continuity between subspaces, these color gamut compressing techniques adopt the morphing procedure commonly employed in computer graphics.
Unexamined Published Japanese Patent Application (Kokai) Nos. 196675/1992 and 196676/1992 disclose color gamut compressing techniques in which lightness is compressed in accordance with the ratio of spread in lightness along the achromatic color axis between the ranges of color reproduction for input and output sides whereas chroma is not subjected to any conversion in the area common to the ranges of color reproduction for input and output sides but compressed in the marginal areas in accordance with the spread of chroma in the ranges of color reproduction for input and output sides.
In addition, Unexamined Published Japanese Patent Application (Kokai) No. 288662/1986 discloses a color gamut compressing technique in which both lightness and chroma are compressed uniformly with specified functions when the range of output color reproduction is narrower than the range of input color reproduction.
The color processing system shown in FIG. 9 is capable of calorimetric reproduction (as faithful as can be seen to the eye); however, if calorimetric reproduction is the only process that is performed, all the colors outside the output color gamut are lost and gradations must be created by compression. On the other hand, the fidelity of colorimetric reproduction is compromised if the color gamut to be reproduced is compressed significantly. In other words, colorimetric reproduction and the compression of a color gamut (creation of gradations in highly chromatic colors) are trade-offs.
Such being the case, the compression of a color gamut should be held to a minimum when performing color transformation for the purpose of calorimetric reproduction. However, all of the color gamut compressing techniques disclosed in the above-mentioned prior patents, the color gamut to be reproduced is compressed uniformly in a non-image-dependent manner and even the pixels in the color gamut of the output medium that need not be compressed are subjected to compression. In other words, the color space is compressed uniformly regardless of the image to be reproduced and this eventually produces not only an effective image but also a non-effective image which is impaired in the fidelity of calorimetric reproduction.